1. Field of the Invention
Implementations of various technologies described herein generally relate to methods and systems for processing seismic data for a towed marine survey.
2. Description of the Related Art
The following descriptions and examples are not admitted to be prior art by virtue of their inclusion within this section.
Seismic data signals are typically acquired by measuring and recording data during a land or marine seismic survey. A land seismic survey may be performed by repeatedly firing an impulsive seismic energy source at the surface of the earth and recording the received signals at a set of sensors or receivers. Similarly, a marine seismic survey may be performed by repeatedly firing an impulsive seismic energy source (e.g., air-guns, dynamite, marine vibrators or the like) into the sea or at the seafloor and recording the received signals at a set of receivers.
In a towed marine seismic survey, the receivers are typically disposed on seismic streamers and the source is disposed on a seismic source array. Both, the seismic streamers and the seismic source array, may be towed across the sea by a vessel. The sensors may typically be laterally displaced at regular intervals along the seismic streamers. However, there may be situations where a non-regular distribution of the sensors may be preferred or where the source array and the streamers may be positioned at different depth levels.
During the marine seismic survey, the cycle of firing the source and recording the received signals may be repeated a plurality of times. Vibroseis is a seismic method in which a vibrator is used as an energy source to generate a controlled wavetrain. A vibrator is an instrument which produces mechanical oscillations used as a seismic source for vibroseis. As such, during a seismic survey, the vibroseis seismic source applies a sinusoidal vibration of continuously varying frequencies into the surrounding medium during a sweep period typically lasting several seconds. In upsweeping, the vibroseis seismic source initially applies a low frequency sinusoidal vibration and increases the frequency of the vibration with time. Conversely, in downsweeping, the vibroseis seismic source initially applies the highest frequency sinusoidal vibration and decreases the frequency of the vibration with time. In either case, the frequency usually changes linearly with respect to time.
In order to acquire a conventional seismic record from the vibroseis seismic source, the field record, which consists of the superposition of many long reflected wavetrains, should be correlated with a sweep wavetrain. The correlated record then resembles a conventional seismic record such as those created using an impulsive source, as opposed to a vibroseis source.
The vibroseis technique is usually associated with land seismic surveys, but it may also be used with marine seismic surveys. In order to use the vibroseis technique with marine seismic surveys, the seismic data acquired via the vibroseis sources should be processed to address the fact that the sources move a certain distance during the transmission of its wavetrain.